Optical properties of the Ge$_{14}$Sb$_{29}$Te$_{57}$ and Ge$_{15}$Sb$_{15}$Te$_{70}$ phase-change materials in the far-infrared range

Infrared absorption spectra of chalcogenide alloys of the GST system with the compositions Ge$_{14}$Sb$_{29}$Te$_{57}$ and Ge$_{15}$Sb$_{15}$Te$_{70}$ in the amorphous and crystalline state were measured and analyzed in the range of 20–400 cm$^{-1}$ (0.6–12 THz) at room temperature. Absorption at these frequencies is due to the manifestation of correlated torsional vibrations of structural units of the amorphous alloy and phonon modes of the crystal. The performed assignment of absorption bands and the revealed differences in the IR spectra make it possible to more confidently represent the possible molecular mechanism of reversible amorphous-crystalline transformations in the studied phase-changing materials